What Is Sag?
Simply put, sag is how much your mountain bike’s suspension compresses under static load.
Here’s an example: Let’s say you’re an aggressive downhill rider. Imagine yourself in an aggressive position on the bike, completely geared up, and balancing in a perfect track stand. Your bike’s suspension should be compressed somewhere between 25-33% just by the weight of you and your gear. The static load is your weight, and as long as you don’t bounce around too much, you’ll get an indication of your ideal sag.
For another example, let’s say that you are a new-ish rider and prefer to cruise through nature on your mountain bike, not very focused on getting an adrenaline fix. You ride sitting down and just want to enjoy the scenery. Instead of setting sag while in an aggressive position, you’ll set your sag sitting down, in the same position you’d be in while riding. Of course, holding this position while measuring sag is pretty difficult so having a friend assist by holding the bike stable makes things much easier!
As you can tell, sag completely depends on the individual but there are a few pieces of criteria to keep in mind before you set up your bike.
- Your riding weight - Measure while completely geared up
- Your typical riding position - Measure based on how you ride your bike
- Static load - Carefully compress the suspension only to the sag point and not bouncing too much while getting on or off the bike.
With the basics out of the way, now we can get into how to set up proper sag for your bike!
How will you know when you have the correct sag?
Generally speaking, a mountain bike’s ideal sag falls between 25-33% for rear shocks and 15-20% for forks. These numbers are averages and sag is totally user preference. But what’s actually right for you? Let’s discuss.
Sag preference depends on rider skill, trail style, and discipline. For example, a cross country rider who likes to blast up climbs is going to prefer a much stiffer suspension than a beginner rider trying out flow trails.
To get a general idea of how it works, experienced riders who ride fast, hard, hit large features, and are generally aggressive will ride stiffer suspension (i.e. less sag) to have the bike be as responsive and quick as possible.
Beginner and intermediate riders who are still getting a feel for the bike, prefer a comfortable ride, or are focused more on the adventure than skill will usually enjoy a softer suspension setup (i.e. more sag).
With all that being said, one of the reasons suspension is so fun is because it’s completely customizable to you. Go with what feels good and do some experimenting. If you’re completely new to setting up sag and want to understand the difference between how soft vs. stiff suspension feels, I’ve got a fun exercise for you!
Bring your shock pump and a tape measure and do two laps on one of your favorite trails. On the first lap, set your sag soft - around 35%. On the second lap, set your sag stiff - in the 25% area. When you ride, think about how it feels accelerating, going over bumps, hitting corners, etc. Once you get a feel for how the air pressure affects your ride, go set up your sag based on the results that you found. Sag has an effect on every bit of the ride and by testing out the extremes you may find which direction you prefer.
Without further ado, let’s go set that sag!
How to Set Mountain Bike Sag
To follow along with this tutorial, you’ll need a ruler or tape measure with inch and mm increments, and a shock pump.
Step One: Open up your compression
To open your compression, you’ll need to turn the compression adjusters fully counterclockwise or labeled as “soft” on some products. Compression adjusters are the blue knobs found on forks or shocks. If you can’t find something that looks like what’s pictured below, your suspension may not have any compression.
To open this shock's compression, you need to move the lever fully to the shock's right.
If you like your settings, be sure to write them down so you can go back to them. If you don’t have a clue about your compression preferences, after you set up your sag, check out our next article about compression and how to set yours!
Step Two: Board Your Bike
With your bike ready to go and in your riding gear, it’s time to hop aboard. Setting sag is easiest if you have a friend around. They can hold the handlebars stable while you balance in your typical riding position. If not, leaning against a wall or tree works just fine.
Step Three: Break The Stiction
Once you are balanced in your typical riding position, lightly bounce on the bike a few times to allow the suspension to move freely. Be sure not to hold the brakes - this will affect the way your suspension rests into it’s sag!
Step Four: Set Sag Rings and Dismount
While still on your bike, slide your sag rings against the seal of the shock and fork. If you don’t have a sag ring you can use a lightly tightened zip tie around the stanchion.
Once the sag ring is in place, hold your riding position for a few seconds then carefully dismount the bike, taking care not to compress the suspension.
Step Five: Calculate and Set Your Sag
For your fork, sag is calculated based on travel. If you have a 160mm of travel and want a 15% sag (stiff suspension), you’ll do the following calculation:
160mm x .15 = 32mm Sag
Here’s a look at that formula:
(Fork Travel in mm) x (Desired Sag Percentage as Decimal) = Ideal Sag Measurement
Once you have your ideal sag calculated, pull out a ruler and measure from the top of the seal to the sag ring. If the sag ring is higher than you’d like, add air to the fork using your shock pump. If the sag ring is lower than you’d like, release pressure. Your looking to get as close to your ideal sag measurement as possible!
Calculating ideal sag for rear suspension is a bit more technical but you’ve got this! Instead of measuring the length of exposed stanchion, you need to base your sag off of the shock’s stroke. Stroke is the entire distance the shock lengthens and compresses but unlike forks, it’s a little harder to eye up.
To measure the stroke of your shock you simply need to move the sag ring to the top of the shock then release all of the air. Once released, fully compress the shock to it’s bottom out point. The sag ring will rest on the bottom out point. Now, extend the shock fully and measure from the dust wiper to the top of the sag ring. This is your stroke!
However, be warned...stroke gets very precise, down to the millimeter in some cases! To be sure that you're 100% accurate, I recommend harnessing the power of the internet.
Here’s how to find your shock’s information online:
Fox shocks include a Custom Tune ID, which is a four digit code that has all the information you need. Head here, and type in your code. Your stroke is measured in either inches or mm. In the photo below, I’ve highlighted where your stroke is listed in the product information.
Most RockShox shocks include the length and stroke written on the bottom of the damper body. The stroke is the second number, as seen in the photo below.
You can also go look up your bicycle’s information on Google. I’ve found that VitalMTB keeps great records of older bike specs. Many brands also have a bicycle archive on their website that list the specs of every bike they’ve made. You just have to do a bit of hunting!
If you're still stumped on finding your stroke, you can always call the manufacturer with your shock’s serial number to figure it out.
Once you have your stroke figured out, you can do the following calculation:
(Shock Stroke) x (Desired Sag Percentage as a decimal) = Ideal Sag Measurement
For example, let’s say that you have a shock with a 57mm stroke. You know you want to run your suspension right in the middle, around 29%. Here’s your calculation: 57mm x .29 = 16.53mm ideal sag.
Once you have your ideal sag calculated, pull out a ruler and measure from the top of the shock’s seal to the sag ring. If the sag ring is higher than you’d like, add air to the shock using your shock pump. If the sag ring is lower than you’d like, release air pressure. You’re looking to get as close to your ideal sag measurement as possible.
Step Six: Set Rebound and Compression Settings
Once you have your mountain bike’s sag figured out, it’s a great time to also set your rebound and compression settings! These settings change based on your air pressure, so what felt good before may no longer be right for you.
Compression and rebound are whole topics in themselves, and if you’d like to read more about this, we’ll be publishing more instructionals to the blog over the next few weeks!
By now you should have your bike’s ideal sag ready to go. Great job! You’ve now got your bike all dialed in and ready to rip! Remember, figuring out ideal suspension settings takes time and lots of trial and error. Keep learning and riding and eventually you’ll run into the perfect setup for you.